With recent advances in laser technology, systems of image recording utilizing laser beams have been explored for implementing high-speed recording or high-density, high-image-quality recording. Thus, studies are in progress on image forming systems using laser heat-sensitive recording materials or laser thermal transfer recording materials, for instance, as recording systems in which a laser beam is converted to heat. Furthermore, the rapid spread of computers and progress in electronics, such as improvements in digital image processing technology gave impetus to an active endeavor to develop the so-called computer-to-plate technique (CTP plate making technique), which makes printing plates directly from digital data.
In the system of recording images through conversion of laser beams to heat (laser thermal recording system), a light absorbing material appropriate to the laser wavelength is used to convert the light absorbed to heat to thereby form images. However, unless the laser output is increased markedly, the heat energy required for image formation can hardly be obtained. Therefore, the advent of a light absorbing material with good light-to-heat conversion efficiency has been awaited. In laser thermal recording, semiconductor lasers are generally used which have light emission bands in the near infrared region of 750 nm to 900 nm. Near infrared absorbing materials matching such laser wavelengths generally absorb in the visible region as well and tend to cause objectional coloration of the background. Thus, a near infrared absorber less absorbing in the visible region of the spectrum is desired.
In the CTP plate making technology, known plate making methods are classifiable into the irradiating method using a laser beam, the method comprising writing by means of a thermal head, the method comprising applying a voltage locally by means of a pin electrode, the method comprising forming an ink-repelling or ink-receiving layer with an ink jet, and so forth. Among them, the method using a laser beam is superior in resolution and in the speed of plate making to other techniques, so that various image forming techniques for practicing said method have been investigated.
Further, recently, small-sized, high-output inexpensive semiconductor lasers having light emission bands in the near infrared region (750 nm to 900 nm) have become readily available and are coming to be utilized as exposure light sources in plate making.
There are two types of direct plate making utilizing laser beams, namely the photosensitive type and heat-sensitive type. As the photosensitive plate material, there are known the electrophotographic system using an organic semiconductor (OPC), the silver salt system using a silver salt, and so on. These plate materials require a large-size and expensive equipment for the manufacture thereof and are relatively expensive as compared with the conventional presensitized (PS) plates. There is also the problem associated with the disposal of the used developer.
Heat-sensitive plate materials are disadvantageous in that they are low in sensitivity as compared with the photosensitive type plate materials. Nevertheless, they have been intensively investigated since they can be handled under ordinary interior conditions (in lighted rooms) and the equipment required is small in size and inexpensive.
All heat-sensitive plate materials require a light-to-heat conversion layer for converting light to heat. This light-to-heat conversion layer contains a light-to-heat conversion agent, for example a near infrared absorbing material. It is essential for such a light-to-heat conversion agent to absorb the laser beam used and, for attaining improved sensitivity, it is necessary that both the ability to absorb the laser beam used and the light-to-heat conversion efficiency thereof be sufficiently high.
The light-to-heat conversion agent includes pigment type and dye type agents. Carbon black is generally used as a pigment type agent. While various substances have been proposed as dye type agents, polymethine compounds are in widespread use. For carbon black, there is a wide assortment of lasers to choose from. However, carbon black is generally less efficient to absorb laser beams as compared with dye type substances, thus calling for its uses in an increased amount. A high-level dispersion technique is also required.
In cases where a dye type substance is used, it is necessary that it be highly capable of absorbing the laser beam used and that it be well compatible with other components such as the image forming component and resin binder and well soluble in the solvent employed.
Polymethine compounds have a methine chain linked by conjugated double bonds within the molecule, absorb in a broad range of the spectrum from the visible to the near infrared region (340 to 1,400 nm) and have high extinction coefficients at their absorption maxima. For these and other reasons, polymethine compounds are used in various fields, for example as photosensitive dyes for silver salt photography, photosensitive dyes for electrophotography, dyes for laser recording, or dyes for laser light generation.
Although polymethine compounds are highly capable of absorbing laser beams, they have several problems to be solved: the compound matching the laser beam must be selected and most known compounds are deficient in light stability and poorly compatible with large forming substances and binder resins, among others.
A large number of polymethine compounds are already known and compounds having a ring structure interrupting the methine chain for enhanced durability have been developed. For example, Compound A is disclosed in JP Kokai S63-319191 (page 3, Compound 9) and Compound B in Journal of Organic Chemistry, 60, 2392, Table 1. 
However, Compound A and Compound B both have maximum absorption wavelengths within the range of 785-815 and are not sensitive enough to small-sized high-output lasers having light emissions in the range of 820˜840 nm. Moreover, both Compound A and Compound B are deficient in solvent solubility and compatibility with resins, so that the kind of binder resin that can be used is limited.